1. Field of the Invention
The present invention relates to an electrophotographic cut-paper printer.
2. Description of the Related Art
A conventional transfer guide guides paper to a transfer point by means of a fixed member using a plastic member. As for a gap between the transfer guide and a photosensitive drum, which is an OPC drum, in a case where various types of paper ream weight are supported, a gap of 1 mm or more is required. Accordingly, there have been cases where, in order to improve the transferability of paper having small ream weight, a nip guide mechanism is jointly used to press the paper onto the photosensitive drum from the reverse side of the paper in synchronism with a paper transporting timing.
With a conventional guide member 1 shown in FIG. 2, if consideration is given to booking by a printer, for which there has been an increasingly strong demand, it is necessary to support various types of ream weight and paper sizes. Accordingly, in order to improve the transferability of paper having small ream weight (thin paper), it is necessary to make a gap B at the guide member 1 small and increase the amount of winding (the distance between a distal end of the guide member and a point of intersection (transfer point 5) between the surface of a photosensitive drum 10 and a line connecting the center (not shown) of the photosensitive drum and a transfer wire 7 (of a transfer unit 6) around the photosensitive drum 10. In the case of paper having large ream weight (thick paper), since the gap B at the transfer guide is narrow, frictional resistance at the distal end of the guide member 1 with respect to paper 8 become large, with the result that there has been a drawback in that a transport failure (paper jamming or the like) occurs. In addition, although the surface of the guide member is in some cases provided with Teflon (registered trademark by DuPont) coating to prevent faulty transport, since the Teflon (registered trademark by DuPont) coating has an insulating property, the surface becomes electrically charged during the passage of the paper and attracts the toner, with the result that there has been a drawback in that the smudging of the back side of the paper occurs. Although in a case where support is provided up to thick paper, it is necessary to enlarge the gap between the guide member 1 and the surface of the photosensitive drum 10 and reduce the amount of winding, whereas, in the case of thin paper, since the gap B is made large and the amount of winding is reduced, the contact quality declines, so that there is a drawback in that the transfer characteristic deteriorates.
To prevent the above-described problems, there are cases where a nip guide 9 and an actuating mechanism 12 for pressing the paper against the photosensitive drum 10 from the reverse side of the paper in synchronism with the paper transporting timing are jointly used, as shown in FIG. 3. In this case, however, if paper having a width narrower than the nip guide 9 is printed, since the nip guide 9 and the photosensitive drum 10 are brought into contact with each other, there has been a drawback in that the nip guide 9 is smudged by a very small amount of toner remaining on the surface of the photosensitive drum 10, and back smudge of first several sheets of paper occurs if the paperwidth is subsequently changed and paper having a large width is printed. In addition, since a complex mechanism is added, there are problems in that the cost of a transfer section increases substantially, and that reliability declines.
An object of the invention is to overcome the above-described problems, and an electrophotographic cut-paper printer in accordance with the invention can be realized by using a transfer guide having a paper holding member using an electrically conductive member attached to a guide member by means of a spring member and disposed in a vicinity of a surface of a photosensitive drum, such that the gap becomes narrow for thin paper and the gap widens for thick paper in correspondence with the ream weight of the paper.
It should be noted that, in Japan, by using 70 kg paper as a boundary, paper having smaller ream weight is set as thin paper and paper having larger ream weight is set as thick paper.